The invention relates to sulfur-containing oligosiloxanes which are liquid at xe2x88x9225-100xc2x0 C., processes for their preparation and their use in silica-containing rubber mixtures which can be crosslinked with sulfur.
Mixtures in which polymers are compounded with reinforcing silicas and sulfur-containing silanes have frequently been proposed for the preparation of rubber mixtures which are crosslinked with sulfur and have a low loss factor in mechanical damping.
Tires of low rolling resistance are those that can be produced with such low-damping rubbers. The particular requirements here during preparation of the mixture (viscosity level), the rubber properties additionally necessary, such as abrasion and wet skidding resistance, and the desired crosslinking properties (scorch resistance) impose considerable demands both on the polymers and on the fillers and the crosslinking system.
The preparation and use of sulfur-containing alkylsilanes is prior art, cf. U.S. Pat. No. 4,100,172, DD-A5-299 187, DE-A1-2 856 229, EP-A1-466 066 and EP-A1-731 824.
DE-A-28 37 117 describes the combination of sulfide-containing silanes and/or mercapto- or alkenylalkoxysilanes in silica-containing rubber mixtures as advantageous for increasing the stability to hot air, in particular of EPDM. DE-A 29 33 247 describes the use of siloxanes with SiOH or SiOR groups in rubber mixtures with silica.
U.S. Pat. No. 4, 474,908 describes the combination of a crosslinking-active with a crosslinking-inactive methylalkoxysilane in order to improve the viscosity and scorch of rubber mixtures.
JP-B-62 48 116 describes rubber mixtures of polymer, carbon black and silicas treated with methylhydrogenosiloxanes and sulfur-containing silanes.
EP-A2-761 748 describes the improvement in the viscosity properties of silica-containing mixtures by admixing siloxanes with hydrogen atoms and alkoxy- and acyloxy groups and optionally, sulfur-containing silanes. The chain lengths of the silanes here is at a degree of polymerization of approx. 40. The ratio of alkenyl- or hydrogeno-siloxane radicals to methylalkoxysiloxane radicals is between 10:90 to 21:79. EP-A2-784 072 also describes mixtures of siloxanes and sulfur-containing silanes in order to lower the viscosity of the mixture.
WO 96/16125 and WO 99/02580 also describe functionalized polyorgano-siloxanes and their production and their use in rubber mixtures, it being possible for the siloxane chains to have a length of up to 300 siloxy units. Furthermore, sulfur-functional polyorganosiloxanes, their production and rubber mixtures containing the same are described in EP-A2 964021 and EP-A2 997489.
Thus, It is known to employ mixtures of siloxanes and sulfur-containing silanes. This has the following disadvantages: first, the substances must be blended very accurately by the customer by a mixing step, and next, large amounts of undesirable alcohols are released during processing due to the alkoxy and acyloxy radicals of the siloxanes, which is ecologically unacceptable.
Furthermore, it is known to produce sulfur-containing oligosiloxanes. This does, however, have the following disadvantages: the direct production from alkenyl oligosiloxanes and sulfur by the known sulfurization processes results in highly viscous and/or gelled products. Other processes are considerably more complicated.
Therefore, the object of the present invention is to provide a mixture of alkoxysiloxane and sulfur donors which is easy to meter, can be mixed homogeneously and at least, in part, does not have the disadvantages of the prior art.
This object is achieved according to the present invention by sulfur-containing siloxanes of the general formula (I) 
wherein
R and Rxe2x80x3 independently of one another represent a C1-C24-alkyl radical or a C6-C24-aryl radical,
Rxe2x80x2 represents a sulfur-containing 2-(p-methylcyclohexyl)propyl radical, a sulfur-containing 2-cyclohexylethyl radical, a sulfur-containing 2-norbornylethyl radical, a sulfur-containing 2-norbornylpropyl radical, a sulfur-containing C4-C24-alkyl radical or a sulfur-containing dicyclopentyl radical,
Rxe2x80x2xe2x80x3 represents R, OR or H, wherein the radicals R and Rxe2x80x2xe2x80x3 can be identical or different,
RIV represents R, SiR3xe2x80x2xe2x80x3 or H, wherein the radicals R, Rxe2x80x2xe2x80x3 and RIV can be identical or different,
the sum of x and y is a number in the range from 2 to 200, with the proviso that always only one radical Rxe2x80x2 is present per siloxane molecule.
xe2x80x9cSulfur-containingxe2x80x9d means that the corresponding radicals have been formed by reaction of a double bond with sulfur and/or hydrogen sulfide. The sulfur-containing radicals, thus, carry xe2x80x94SH, Sx or other sulfur substituents. These can, optionally, also be coordinated associatively on the double bond. Sx here denotes sulfur chains or rings with a length in the range of 1-100 sulfur atoms.
If Rxe2x80x2 represents a sulfur-containing C4-C24-alkyl radical, the C4-C24-alkyl represents e.g. a butane, pentane, hexane, heptane, octane or nonane radical, preferably a butane, hexane or octane.
R and Rxe2x80x3 represent a C1-C24-alkyl radical, which can be present in a linear, branched or also a cyclic structure, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, cyclopentyl, cyclohexyl or tert-butyl radicals. For R and Rxe2x80x3, methyl, ethyl, propyl, cyclopentyl, cyclohexyl and tert-butyl radicals are preferred, more preferably methyl, ethyl, cyclohexyl and tert-butyl radicals. The alkyl radicals can also be halogen-substituted, and Cl-methyl and Cl-propyl radicals are preferred.
R and Rxe2x80x3 can also represent a C6-C24-aryl radical, which can be substituted in its turn by the above-mentioned C1-C24-alkyl radicals or other aryl radicals, such as the phenyl, cyclopentadienyl, naphthyl, methyl-phenyl, ethylphenyl or tert-butylphenyl radical. Phenyl radicals are preferred.
The above-mentioned alkyl radicals can, of course, in their turn also be substituted again by aryl radicals, such as phenylmethyl, phenylethyl or also triphenylmethyl radicals.
The sum of x and y is a number in the range from 2 to 200, preferably 2 to 50, more preferably 2 to 20, and most preferably 2 to 10.
The x elements of the structure [RRxe2x80x2SiO] and the y elements of the structure [ORxe2x80x3Rxe2x80x2xe2x80x3SiO] can, of course, each be arranged as blocks along the siloxane chain in a sequential fashion or regularly or in random distribution.
If RIV=H or OR, cyclic condensates can be formed. If RIV=H at both chain ends, such condensates are formed spontaneously in the mixture with the elimination of water. If at one chain end, RIV=H and at the other chain end, RIV=OR, such condensates are formed spontaneously in the mixture with the elimination of the alcohol ROH. If RIV=OR at both chain ends, such condensates are formed in the mixture in the presence of catalysts and/or water, e.g. atmospheric moisture, with the elimination of the alcohol ROH.
The problem is also solved according to the present invention by sulfur-containing siloxanes (Ia) which are composed of the following structural units (K), (L), (M) and (N) 
wherein
RVI represents a C1-C24-alkyl radical, a C6-C24-aryl radical, a C1-C24-alkoxy radical, a C6-C24-aryloxy radical, H or OH,
Rxe2x80x2 has the abovementioned meaning,
RVII, RVIII, RIX, RX, RXI and RXII independently of one another represent a C1-C24-alkyl radical, a C6-C24-aryl radical or H,
the above-mentioned restrictions apply to x and y and
w and z each independently of one another can be an integer between 0 and 100. The individual structural units can be arranged in succession in any desired order and linearly or cyclically. If a linear chain is present, terminal chain groups RIV and ORIV can additionally be present, wherein RIV has the abovementioned meaning.
The sulfur-containing siloxanes of the structure (I) according to the present invention can be present either as pure compounds or as mixtures of various compounds. In addition, the sulfur-containing siloxanes of the structure (Ia) according to the present invention can be present either as pure compounds or as mixtures of various compounds. It is, of course, also possible for mixtures of siloxanes of the structure (I) and those of the structure (Ia) to be present.
The sulfur-containing siloxanes according to the present invention can be prepared analogously to known processes, as described in U.S. Pat. No. 4,100,172 or DE-A-4 435 31 1.
However, the sulfur-containing siloxanes according to the present invention can advantageously be prepared from siloxanes of the general formula (II) and/or (IIa) 
wherein
R, Rxe2x80x3, Rxe2x80x2xe2x80x3, RIV, RVI, RVII, RVIII, RIX, RX, RXI and RXII represent the radicals already mentioned and w and z have the above mentioned meanings and
RV represents a limonyl radical, an ethylenecyclohexene radical, an ethylenenorbornenyl radical or norbornylethylidene or norbornylvinyl radical, a C4-C24-alkenyl radical or a bicyclopentenyl radical and
the restriction already defined applies to the sum of x and y, with the proviso that always only one radical RV is present per molecule, the siloxanes of the general formula (II) and/or (IIa) being reacted with elemental sulfur or with a mixture of sulfur and hydrogen sulfide in the presence of a catalyst.
The sulfur-containing siloxanes of the structure (II) according to the present invention can be present either as pure compounds or as mixtures of various compounds. It is also possible for the sulfur-containing siloxanes of the structure (IIa) according to the present invention to be present either as pure compounds or as mixtures of various compounds. It is, of course, also possible for mixtures of siloxanes of the structure (I) with those of structure (Ia) to be present.
The elemental sulfur is employed in amounts in the range from 1 to 8 mol of sulfur per mol of RV, preferably in amounts in the range from 1 to 6 mol of sulfur per mol of RV, more preferably 1 to 4 mol of sulfur per mol of RV.
If mixtures of sulfur and hydrogen sulfide are used, these are likewise employed in amounts of 1 to 8 mol of total sulfur per mol of RV, preferably 1 to 6 mol of total sulfur per mol of RV and more preferably 1 to 4 mol of total sulfur per mol of RV. The ratio of sulfur to H2S is 1:0.01-1, preferably 1:0.2.
Amines, mercaptobenzothiazole, salts as described in WO087/00833 p. 6, l.24 et seq., disulfur dichloride or other catalysts known to the expert which catalyze the addition of sulfur on to double bonds can be used as the catalyst cf. EP-A2-531 842, EP-A1-25 944, DE 2 111 842. However, amines, such as tertiary C12-C14-amines or mercaptobenzothiazole or disulfur dichloride are preferably used.
The catalyst is employed in amounts in the range from 0.001 to 0.1 mol per mol of sulfur, and preferably, 0.005 to 0.01 mol per mol of sulfur. Mixtures of catalysts can, of course, also be employed.
The reaction of the siloxanes (II) and/or (IIa) with sulfur or sulfur/H2S in the presence of a catalyst can be carried out in any suitable apparatus known to one skilled in the art. It is advantageous to ensure good thorough mixing.
The reaction is carried out at temperatures in the range from 0 to 200xc2x0 C., preferably 120-180xc2x0 C., more preferably 120-160xc2x0 C.
The reaction is carried out under pressures in the range from 0 to 100 bar, preferably 0-30, more preferably 0.5-15.
The present invention also provides both the siloxanes of the general formula (II) and/or (IIa) described and the processes described for their reaction with sulfur or sulfur/H2S.
The siloxanes of the formula (II) and/or (IIa) according to the present invention can, in principle, be prepared by customary methods described in the literature. See, for example, EP-A1 2 59 625, EP-A1 4310 and EP-A1 3 514.
The siloxanes of the general formula (II) according to the present invention can advantageously be prepared from siloxanes of the general formula (III) 
wherein the radicals have the meaning already mentioned and
x+y represents a number in the range of 2-200, preferably 2-50, more preferably 2-20, and most preferably 2-10.
Accordingly, the siloxanes of the general formula (IIa) according to the present invention can advantageously be prepared from siloxanes of the general formula (IIIa) 
wherein
the radicals have the meanings already mentioned and the restrictions mentioned apply to x+y, z and w.
The siloxanes (III) and/or (IIIa) are reacted here with alcohols and dienes in the presence of a catalyst.
Any alcohol known to the expert is possible here, in principle, as the alcohol, and examples which may be mentioned are C1-C24-alcohols, such as methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, decanol, phenol, in particular methanol, ethanol, propanol and butanol, very particularly preferably ethanol and propanol.
The dienes corresponding to RV are employed as the diene. These are limonene, vinylcyclohexene, ethylidenenorbornene, vinylnorbornene and C4-C24-dienes, preferably, butadiene, hexadiene, heptadiene, octadiene and bicyclopentadiene.
All the catalysts known to the expert which catalyze the substitution of hydridic hydrogen atoms on silicon atoms by dienes are possible as the catalyst, in particular platinum, rhodium, ruthenium or nickel, and salts and/or complexes thereof.
The catalyst is employed in amounts in the range of 1-100 ppm of metal, based on the total amount of diene and siloxane, preferably 5 to 500 ppm, and most preferably 10-50 ppm.
The ratio of ORxe2x80x3 to RV is determined by the stoichiometric ratio of the alcohol to diene and the sequence of addition of the educts.
The siloxanes (III) and/or (IIIa) are preferably reacted first with the alcohol and then with the diene. Furthermore, the siloxanes (III) are preferably reacted simultaneously with the alcohol and the diene.
It is, of course, also possible to employ mixtures of different alcohols and/or mixtures of different dienes.
The reaction is carried out at temperatures in the range from xe2x88x9220 to +200xc2x0 C., preferably 20-100xc2x0 C.
The reaction is carried out under pressures of 0 to 50 bar, preferably 0-5 bar.
The reaction can be carried out in any apparatus suitable for hydrosilylation reactions which is known to one skilled in the art.
The reaction products are optionally purified, for example, by distillation, optionally under reduced pressure or by another suitable process, in order to remove excess, non-reacted substances and secondary products.
The present invention also provides the use of the sulfur-containing siloxanes of the general formula (I) and/or (Ia) and of the siloxanes of the general formula (II) and/or (IIa) in silica-containing rubber mixtures.
Additionally, the present invention also provides a mixture of the sulfur-containing siloxanes of the general formula (I) according to the present invention with siloxanes and/or sulfur-containing silanes of the prior art. The siloxanes and/or sulfur-containing silanes described in U.S. Pat. No. 4,100,172, EP-A 1 466 066, DE-A 2 837 117, DE-A 2 933 247, U.S. Pat. No. 4,474,908, JP-A 6 248 116, EP-A 1 761 748 and EP-A 1 784 072 e.g. are possible here.
The sulfur-containing siloxanes (I) and/or (Ia) can also be employed as a mixture with the siloxanes (II) and/or (IIa).
If the siloxanes of the general formula (II) and/or (IIa) are used without the sulfur-containing siloxanes of the general formula (I) and/or (Ia), sulfur-containing alkoxysiloxanes, which have in the molecule at least one alkoxy radical which is capable of reacting with the Sixe2x80x94OH groups of the silica surface under the mixing conditions and which carry in the molecule at least one sulfur-containing radical which is capable of reacting with the unsaturated rubber under the mixing or vulcanization conditions must furthermore be employed.
Preferred sulfur-containing alkoxysiloxanes in this case are, in particular, bis-(trialkoxysilyl-alkyl) polysulfides, as described in DE 2 141 159 and DE-AS 2 255 577, and oligomeric and/or polymeric sulfur-containing alkoxysilanes of DE-OS 4 435 311 and EP-A 670 347.
The sulfur-containing alkoxysilanes are then employed in amounts of 0.1 to 20 parts by wt., preferably 0.5 to 10 parts by wt., based on 100 parts by wt. of rubber.
Silicas, which are employed for the silica-containing rubber mixtures are:
silicas prepared by precipitation of solutions of silicates with spec. surface areas of 30 to 1,000, preferably 30 to 400 m2/g (BET surface area) and with primary particle sizes of 10 to 400 nm. The silicas can optionally also be present as mixed oxides with other metal oxides, such as oxides of aluminum, magnesium, calcium, barium, zinc, zirconium or titanium;
silicates, e.g. aluminum silicate and alkaline earth metal silicates, such as magnesium silicate or calcium silicate, with BET surface areas of 30 to 400 m2/g and primary particle diameters of 10 to 400 nm.
Such products are described in more detail, for example, in J. Franta, Elastomers and Rubber Compounding Materials, Elsevier 1989, pages 401-447.
Suitable rubbers are, in addition to natural rubber, also the known synthetic rubbers. They include, inter alia, polybutadiene, butadiene/acrylic acid C1-C4-alkyl ester, polychloroprene, polyisoprene and polyisoprene copolymers, styrene/butadiene copolymers with styrene contents of 1 to 60, preferably 20 to 50 wt. %, styrene/butadiene copolymers with 1-20 wt. % of further polar unsaturated monomers, in particular styrene/butadiene/acrylonitrile copolymers with styrene contents of 1 to 40% and acrylonitrile contents of up to 20%, isobutylene/isoprene copolymers, butadiene/acrylonitrile copolymers with acrylonitrile contents of 5 to 60, preferably 10 to 40 wt. %, partly hydrogenated or completely hydrogenated NBR rubber, ethylene/propylene/diene copolymers and mixtures of these rubbers.
Natural rubber, BR, SBR and styrene/butadiene/acrylonitrile copolymers are preferred, in particular, for the production of motor vehicle tires with the aid of the sulfur-containing siloxanes of the general formula (I) according to the present invention or siloxanes of the general formula (II) according to the present invention.
Preferred ratios of rubber to silica are 100:10 to 100:150, more preferably 100:20 to 100:100.
Carbon blacks and the conventional rubber auxiliaries, such as e.g. stabilizers, mold release agents, plasticizers etc., can moreover be added.
The amounts of rubber auxiliaries added depends on the particular intended use. Preferred amounts of carbon blacks are 0 to 30 parts by wt., amounts of stabilizers are 0.1 to 1.5 parts by wt., and amounts of plasticizers are 5 to 75 parts by wt. per 100 parts by wt. of rubber. Mineral oil plasticizers are to be understood as meaning paraffinic, naphthenic or aromatic mineral oils with VDC numbers (viscosity-density constants) of 0.791 to 1.050, preferably 0.85 to 1.0, and refraction intercepts Ri of approx. 1.048 to 1.065.
Such mineral oil plasticizers are commercially obtainable. Aromatic mineral oil plasticizers are preferred plasticizers.
The rubber mixtures can be prepared in a conventional manner, e.g. by means of kneaders, roll mills or extruders.
The rubber mixtures can optionally also comprise further fillers, such as
naturally occurring silicates, such as kaolin and other naturally occurring silicas;
glass fibers and glass fiber products (mats, strands) or glass microbeads;
metal oxides, such as zinc oxide, calcium oxide, magnesium oxide or aluminum oxide;
metal carbonates, such as magnesium carbonate, calcium carbonate or zinc carbonate;
metal hydroxides, such as e.g. aluminum hydroxide or magnesium hydroxide;
carbon blacks; the carbon blacks to be used here are prepared by the flame black or furnace or gas black process and have BET surface areas of 20 to 200 m2/g, such as e.g. SAF, ISAF, HAF, FEF or GPF carbon blacks.
Highly dispersed precipitated silicas and carbon blacks are preferably employed. The fillers mentioned can be employed by themselves or as a mixture.
Moreover, further rubbers can be admixed to the rubber mixtures in a conventional manner: Natural rubber, emulsion SBR and solution SBR rubbers with a glass transition temperature above xe2x88x9250xc2x0 C., which can optionally be modified with alkoxysilane or other functional groups, as described e.g. in EP-A 447 066, polybutadiene rubbers with a high 1,4-cis content ( greater than 90%), which are prepared with catalysts based on Ni, Co, Ti or Nd, and polybutadiene rubbers with a vinyl content of 0 to 75% and mixtures thereof are of interest in particular for the production of motor vehicle tires. Solution SBR rubbers with a vinyl content of 20 to 60 wt. % and polybutadiene rubbers with a high 1,4-cis content ( greater than 90%) are preferably employed.
The rubber mixtures can, of course, also additionally comprise further rubber auxiliary products which are known and conventional in the rubber industry, such as reaction accelerators, anti-aging agents, heat stabilizers, light stabilizers, ozone stabilizers, processing auxiliaries, plasticizers, tackifiers, blowing agents, dyestuffs, pigments, waxes, extenders, organic acids, retardants, metal oxides and activators, such as triethanolamine, polyethylene glycol and hexanetriol. The rubber auxiliaries are admixed in the conventional amounts and depend on the particular intended use envisaged. Conventional amounts are, for example, amounts of 0.1 to 50 wt. %, based on the total amount of rubber employed.
In addition to the above-mentioned rubber auxiliary products, the known crosslinking agents, such as sulfur, sulfur donors or peroxides, can be added to the rubber mixtures according to the present invention. The rubber mixtures according to the present invention can, moreover, comprise vulcanization accelerators, such as mercaptobenzothiazoles, mercaptosulfenamides, guanidines, thiurams, dithiocarbamates, thioureas and/or thiocarbonates. The vulcanization accelerators and the crosslinking agents mentioned are conventionally employed in amounts of 0.1 to 10 wt. %, preferably 0.1 to 5 wt. %, based on the total amount of the particular rubber employed.
The vulcanization of the rubber mixtures according to the present invention can be carried out at conventional temperatures of 100 to 200xc2x0 C., preferably 130 to 180xc2x0 C. (optionally under a pressure of 10 to 200 bar).
Further blending of the rubbers with the other rubber auxiliary products, crosslinking agents and accelerators mentioned can be carried out in a conventional manner with the aid of suitable mixing units, such as roll mills, internal mixers and mixing extruders.
The rubber vulcanization products, which can be prepared from these, are suitable for the production of all types of shaped articles, e.g. for the production of cable sheathings, hoses, drive belts, conveyor belts, roller coverings, shoe soles, sealing rings and damping elements. They are particularly suitable for the production of tires, since such tires have a particularly low rolling resistance, a particularly good wet skidding resistance and a high abrasion resistance.
The following examples illustrate the invention in more detail.